1. Field of the Invention
The invention relates generally to the field of radio systems, and more specifically to methods and apparatus for monitoring trunked radio communications, decoding the digital data from trunked radio control channels, and to methods and apparatus for determining the type of trunked radio system by observing the characteristics of the digital data transmitted on a control channel.
2. Description of the Related Art
Modernly, the two types of mobile radio communication systems. The first is a conventional radio communication system. Here, mobile radio units are assigned a single radio frequency channel. All subscribers to the system share the same channel, and cooperate by monitoring the channel for activity. Most conventional radio communication systems work well when there is a small number of subscribers and the usage patterns for all the subscribers is similar.
To overcome the limitations of the conventional radio communications system, an implementation of a trunked communications system is employed. In the trunked communication system, a central controller allocates a number of channels among many subscribers. Generally, the subscribers are organized into fleets, sub-fleets or groups, and individuals. Accordingly, each subscriber has a fleet, group, and an individual identification code. A mobile radio initiates communications by transmitting a digital request message to a system central controller via a predetermined inbound frequency. Here, the mobile unit generates a channel request that includes the necessary information to identify it to the central controller. The central controller responds to the mobile unit with a digital message via a separate and distinct predetermined outbound frequency that, in turn, authorizes the mobile unit to operate on an n another channel. This outbound digital message is often referred to as an outbound signaling word (OSW) or a channel assignment message or a channel grant message. Both the inbound and outbound channel is often referred to as a “system control channel”. In addition, the mobile unit also responds to other commands that are received from the central controller.
There are a number of trunked radio systems that are currently use in the United States. One of these trunked radio systems is described in U.S. Pat. Nos. 4,692,945 and 4,723,264 issued to Motorola. These two patents disclose an improved dispatch trunked radio system having an expanded unifying signaling protocol used to implement several enhanced system features. Here, the subscriber or mobile units are capable of generating and receiving signaling information in a single word or concatenated in a dual-word signaling scheme. The central control unit is able to decode and generate signaling information in either a single or concatenated dual-word signaling scheme, as well as generates channel controlling information in accordance with either signaling scheme.
In addition, U.S. Pat. Nos. 4,055,832 and 4,312,070 also issued to Motorola describes the method used to encoding data transmitted over the control channel of a trunked radio system implemented by Motorola. Here, En and Coombes, et al. discloses an encoding and decoding system suited for use in a mobile trunked dispatch communication system capable of error correction, error detection, and detection of loss of synchronization. More specifically, the system disclosed in En and Coombes, et al., respectfully, provides an error correction system that can correct one random error out of every four bits transmitted with a minimum of time delay in transmission and a minimum number of components for system implementation.
In U.S. Pat. No. 4,905,302 issued to Childress, et al. discloses another trunked radio communications system, known as EDACS, that provides substantial improvements in channel acquisition and channel drop, and in reliability of critical control signaling. The system uses a much higher digital signaling rate than is typically found in prior art systems, and uses a control channel to convey digital channel request and assignment messages between the central site and mobile transceivers. The mobile radio systems transmit channel requests on the inbound control channel (if no response is received, the mobile retries during a retry time window which increases in duration in dependence on the number of retries). The mobile transceiver switches to a working channel in response to an assignment message received on the control channel. Digital signals transmitted on the control channel and subaudible digital signal transmitted on active working channels allow late entry, shifting to higher priority calls, and other advanced functions. Message and transmission trunking capabilities are both present so as to maximize working channel usage without compromising channel access for high priority communications. During transmission, the calling as well as the calling transceivers return to the control channel after each transmission (and called transceivers may be inhibited from transmitting) but grant higher priority to calls from the other transceivers being communicated with to ensure continuity over an entire conversation.
It is desirable to monitor various users of a communication system, but without any capability of transmitting. Conventional communication systems are easily monitored with a variety of radio communication receivers. One example of this type of radio communication receiver is a scanner. Scanners are radio receivers that typically cover a range of frequencies between approximately 30 MHz and 1 GHz. The scanning receiver is sequentially tuned through a set of predetermined, user selected frequencies. If a signal is detected, the scanner stops scanning and remains tuned to that frequency until the signal ceases, whereupon the scanner resumes searching for another frequency. The typical scanner is comprised of a receive-only radio without any capability of transmitting a signal. Such scanning receivers are inexpensive and enable a user to hear transmissions on a conventional radio system with little effort on the part of the user, and little or no effort on the part of the communications systems operator. A conventional scanner is not able to correctly follow a sequence of transmissions on a trunked radio system because the scanner has no ability to understand the outbound channel assignment messages sent by the trunking system controller.
U.S. Pat. No. 5,784,388 issued to Knox discloses several significant improvements to a conventional scanner and improves the ability to decode the messages from a Motorola trunked system control channel. The system disclosed in Knox decodes encoded data from the control channel of a trunked radio communications system. The decoder includes a de-interleave, an auto synchronization sequence combiner, and a table lookup error detector to recover the transmitted information and also indicates whether any errors are present in the recovered information. However, the advancement disclosed by Knox requires that the scanner be specifically designed and manufactured to operate for use with Motorola trunked radio communications systems. Additionally, the system disclosed by Knox requires that the operator of the scanner be able to identify the various types and characteristics of a trunked system, the particular radio frequencies of the system as well as to program these details into the scanner or computer.
In the case of the EDACS trunking system, the data messages transmitted by the system controller do not contain frequency information. EDACS messages only contain logical channel numbers. EDACS mobile units are pre-programmed with a frequency table that allows the mobile unit to associate a specific radio frequency with logical channel numbers. To properly tune a scanning radio to an EDACS working channel, the user must know which frequencies are assigned to the system.
Therefore, there is a need for providing a method and apparatus for monitoring trunked radio communications system that automatically determines the characteristics and frequencies of trunked radio communications systems, thus, eliminating the need for a user to program these details.